Data transmission systems are used in diverse forms, for example data packets are intended to be interchanged via radio over short distances. Such a data transmission over short distances is provided inter alia in the event of data interchange between the base part and the mobile part of a cordless telephone or between a computer and peripherals. Such data transmission systems in which data are interchanged in wire-free fashion over short distances of just a few meters between a base station and mobile stations are referred to as piconetwork. Piconetworks can be operated according to different standards, such as, for example, the Bluetooth or DECT standard (Digital European Cordless Telecommunications).
Time slot methods are usually used for the data transmission. In time slot methods, downlinks (data transmission from the base station to the mobile station) and uplinks (data transmission from the mobile station to the base station) are allocated time slots with a specific temporal length. Time slot methods that are frequently used are the TDMA method (Time Division Multiple Access) as a multiple access method and also the TDD method (Time Division Duplex) as a duplex method for the formation of a bidirectional channel between the base station and the mobile stations.
In order to ensure a narrowband transmission range within a frequency channel, a two-valued GFSK modulation method (Gaussian Frequency Shift Keying) is used for example in digital cordless communication systems based e.g. on the Bluetooth standard. The GFSK modulation method uses a Gaussian filter for limiting the frequency bandwidth, thereby suppressing crosstalk between the individual frequency channels.
At the present time, in digital cordless communication systems based for example on the Bluetooth standard, data are transmitted at rates of 1 Mbit/s as standard with application of frequency hopping methods. One possibility for achieving higher data transmission rates is to use higher-valued modulation methods, such as, for example, the π/4-DQPSK (Differential Quadratur Phase Shift Keying), π/8-DQPSK or M-FSK method (Frequency Shift Keying), in which an M-valued symbol where M=2m is transmitted instead of a (two-valued) bit.
In many mobile radio systems, it is possible to change over between two modulation methods, for example GFSK and DQPSK, within an existing radio link in order thus to obtain a higher data transmission rate. In the case of a mobile radio system that uses the TDMA time slot method, for example, for data transmission, such a changeover between the modulation methods may be effected within a burst. In addition to a higher data transmission rate, it is also possible in this way to ensure a desirable downwards compatibility with modulation methods used in earlier versions of a communications standard.
In the case of such mobile radio systems, it may be provided that a first modulation method is used for setting up a connection between a base station and a mobile station. A transmission burst comprises a first data burst header or data packet (header), which is transmitted by the first modulation method and indicates to the respective remote station with which a radio link is set up that a changeover is to be made to a second modulation method. However, a changeover is effected only when the remote station also supports this second modulation method. Such control of the changeover between the modulation methods may also be performed at higher protocol levels. In the event of the receiver being changed over to the second modulation method a few components, such as reception filters for example, are generally changed over, as a result of which changeover effects, such as propagation time changes in the signals, for example, may occur in the reception signal path. On account of the group delay times dependent on the filter properties, it is therefore necessary to carry out a renewed symbol synchronization with the data transmitted by the second modulation method. This further symbol synchronization is carried out with the aid of a second data burst header that is contained in the transmission burst and is transmitted by the second modulation method. This renewed synchronization process generally requires an additional synchronization unit in the respective receiver, as a result of which the outlay on components is relatively high.
Generally, a symbol synchronization comprises the two phases of acquisition and tracking. Depending on the burst structure, in particular on the length of the synchronization word (sync word) or access code in the data burst header, the acquisition has to be effected relatively rapidly. This can generally be realized only with a relatively high technical outlay. A second synchronization process within a burst is therefore problematic both with regard to the loss of time and with regard to the hardware outlay required therefor.